The heart requires precise coordination of its mechanical and electrical behavior to function optimally. The human body normally regulates cardiac output in response to body needs by changing the heart rate, as during physical exercise, and/or by adapting the stroke volume. Under pathological conditions, however, some of the normal regulatory mechanisms may be damaged. For example, heart tissue damaged due to myocardial infarct typically cannot sustain normal pumping function. Alternatively or additionally, normal electrical signals are not generated, or are impaired in their propagation, such that cardiac output and cardiac efficiency (stroke work divided by oxygen consumption) are correspondingly compromised. Standard pacemakers known in the art are able to control the rate of the heart, e.g., to accelerate the heart rate after detecting bradycardia, but are not able to increase contraction strength over the long-term without producing adverse side-effects.
PCT Patent Publication WO 97/25098, to Ben-Haim et al., entitled “Electrical muscle controller,” and the corresponding U.S. patent application Ser. No. 09/101,723, which are assigned to the assignee of the present patent application and are incorporated herein by reference, describe methods for modifying the force of contraction of at least a portion of a heart chamber by applying a non-excitatory electric signal to the heart at a delay after electrical activation of the portion. The non-excitatory signal is such as does not induce action potentials in cardiac muscle cells, but rather modifies the cells' response to the activation. In the context of the present patent application, the use of such a non-excitatory signal is referred to as Excitable-Tissue Control (ETC). The non-excitatory signal may be applied in combination with a pacemaker or defibrillator, which applies an excitatory signal (i.e., pacing or defibrillation pulses) to the heart muscle.
PCT Patent Publication WO 98/10832, to Ben-Haim et al., entitled “Cardiac output enhanced pacemaker,” and the corresponding U.S. patent application Ser. No. 09/254,900, which are assigned to the assignee of the present patent application and incorporated herein by reference, describe a pacemaker that gives cardiac output enhancement. This pacemaker applies both excitatory (pacing) and non-excitatory (ETC) electrical stimulation pulses to the heart. By applying non-excitatory pulses of suitable strength, appropriately timed with respect to the heart's electrical activation, the contraction of selected segments of the heart muscle can be increased or decreased, thus increasing or decreasing the stroke volume of the heart.